The inventive concept relates to a fabrication method for nonvolatile memory devices. More particularly, the inventive concept relates to a method of fabricating nonvolatile memory devices having a variable resistance material.
Nonvolatile memory devices using resistance materials include Phase Change Random Access Memory (PRAM), Resistive RAM (RRAM), and Magnetic RAM (MRAM). Unlike Dynamic RAM (DRAM) that stores data as charge placed on a capacitor, flash memory devices use various materials having changeable properties to store data. For example, PRAM uses a phase-changeable material such as a chalcogenide alloy, RRAM uses a variable resistance material, and MRAM uses Magnetic Tunnel Junction (MJT) thin films having a variable magnetization state.
As one example of a nonvolatile memory device using a variable resistance material, the PRAM will be described in some additional detail. The phase-change material used in conventional PRAMs has a crystalline state exhibiting low resistance and an amorphous state exhibiting high resistance. The crystalline state is commonly assigned a corresponding data value of 0, and the amorphous state is assigned a data state of 1.
The conventional PRAM is programmed (set or reset—assuming a binary memory cell) with a write pulse (e.g., a set pulse or a reset pulse). The write pulse applied to the phase-change material during a write operation uses a joule heating effect to define the desired data state (i.e., the crystalline or amorphous data state). When writing a data value of 1, the PRAM uses the reset pulse to heat the phase-change material above the melting temperature and then allows rapid cooling to define the amorphous state. When writing a data value of 0, the PRAM uses the set pulse to heat the phase-change material above the crystallization temperature and below the melting temperature and then maintains the heating temperature for a predetermined time to define the crystalline state.